Evaluation of dew point corrosion in acid atmosphere by wire beam electrode

In this article, wire beam electrode (WBE) was used to evaluate the corrosion behavior of ND steel in environmental acid atmosphere with different partial pressure of CO2. Meanwhile, corrosion products and surface morphology analysis also used to support this research. The results showed that the corrosion behavior began from the edge of droplet in dew point corrosion, and gradually spread to the center of it. The spread speed would increasing with CO2 partial pressure enhance, which was 24h in 5% CO2 and 4h in 50% CO2. Corrosion current density in the edge of droplet can form the “cathode-anode-cathode ring" structure, and disappears gradually as the corrosion time was going. Corrosion morphology observation results showed three ring shapes region and different elemental composition of different corrosion products, which is correspondence with the "cathode-anodWe-cathode ring" structure measured in WBE experiments. The results showed that the reaction gradually transferred to the uniform corrosion on electrode surface when the dew point corrosion reaction reaching the late stage. It comes from the dissolution, diffusion and reaction of gaseous corrosion medium of CO2 and O2.


Introduction
Dew point corrosion (DPC) is the most important type of high temperature equipment corrosion [1][2] . Some researchers' consideration that DPC is a kind of special corrosion in thin electrolyte film, and the corrosion rate was due to the rate of corrosion media mass-transfer process. As for DPC, the corrosion media is different to the common thin electrolyte film, the acid atmosphere such as SO 2 , SO 3 , CO 2 and HCl are became the corrosion media instead O 2 to attack the base metal [3][4] .

Materials preparation
The metal we chose is 09CrCuSb steel (ND steel), which is a new type of low alloy steel used in DPC environment. The chemical compositions showed TAB 1, referring to standard of GB/T150. 2-2015. As the result showed, the chemical compositions of this steel are kept according to the requirements of the standards, and it used to make the electrodes and observation samples.  1 is the metallographic structure of ND steel. As the results, its microstructure is ferrite with a few pearlite and carbides.

Wire beam electrode measurement
In order to study the transfer mass-transfer characteristics, wire beam electrode (WBE) measurement were used to evaluate the surface current distribution on the ND electrode. The WBE is formed with an11×11 wire bundle electrode arrays (121 electrodes). Every electrode cuts into a cylinder with 1mm surface diameter, and sealed into an epoxy resin cube with 0.3mm interval. The working surface of the WBE was subsequently ground with 150 to 2000 grit emery papers, and then cleans it with distilled water and acetone after polishing. The measurement was performed with a Corttest CT520 WBE scan workstation with the surface current scanning model, and the result presents current distribution on surface.
We used a work-holding device to hold the WBE work surface keep up, and dropped 0.5ml distilled water on it formed a single corrosive droplet. In order to prevent the droplet evaporation, WBE set in a constant temperature and constant humidity chamber with 25℃and 100%RH. The internal dimensions of the chamber (l×w×h) is 100×100×50mm, and inject 5%and 50% CO 2 acid gas mixed by CO 2 and N 2 with flow rate of 100ml/min, making the saturated acid atmosphere. In this experiment, 121 electrodes which coupled together are disconnected successively, and the couples current between each electrode and the other electrodes were measured respectively. Scanning time are 0, 2, 4, 8, 24, 48 hours after corrosion occurrence, totally 6 times. Fig. 2 showed the change of current distribution on ND steel surface in5% CO 2 acid atmosphere. In this experiment, all surface of WBE cell was covered by distilled water droplets, and the abscissa and ordinate coordinates represent different electrode positions. It can be seen that the positions located on the edge and the center of WBE surface showed the lowest distribution current value at 0 h, and it presents those positions became cathodes in this reaction. At the same time, the distribution current close to the edge showed the highest distribution current value, and became anode. The corrosion current formed a structure of "cathode-anode-cathode ring" from the outer edge to the center under droplets, and the position of anode might be the area that corrosion reaction is the most serious. Therefore, the corrosion current reached 0.0265μAat 0h in anode area.  Cathode current at the edge of WBE disappeared gradually as the corrosion time was going, and the position of the anode was not changed basically, but the current value decreases gradually. The maximum anodic current values reached 0.0157, 0.0066 and 0.0059μA at time after corrosion time of 2h, 4h, 8h respectively, which were 41%, 75% and 76% lower than 0h.The corrosion reaction tended to stable at 8h, and the electrode surface could not observe the characteristics of separation of anode and cathode. The corrosion proceeded more smoothly on the surface of the electrode, and the corrosion characteristics were mainly uniform corrosion.   3 showed the change of current distribution on ND steel surface in5% CO 2 acid atmosphere. As the results, the distribution of cathode and anode on WBE surface in 50% CO 2 environment is similar to that in 5% CO 2 environment at the initial stage of corrosion reaction, but it is much more severe. The maximum anodic current reached about 0.0328μA at 0 h, which is about 1.24 times higher than that in 5% CO 2  The current density of the anode decreases gradually as the corrosion time was going, and it is faster than that in 5% CO 2 environment. The separation of anode and cathode has been slight in electrochemical reaction at 4h, and then stabilized at 24h.That means that the corrosion degree on the WBE surface has declined, and it tended to the uniform corrosion. Fig.4 showed the comparison of the maximum current on WBE surface with 5% and 50% CO 2. It can be seen that the maximum surface current of ND steel has similar change tendency in the two different concentration of CO 2 content environment. All of them have the highest surface current at 0 h, which decreases rapidly as the corrosion time was going. 5% CO 2 reached steady state at 8h, while0% CO 2 reached steady state at 24h. The experiment results in 5% and 50% CO 2 atmosphere all point to that the reaction gradually transferred to the uniform corrosion in all beam on the surface of every single electrodes as the corrosion time was going, and the characteristics of the galvanic reaction of the electrode decreased gradually.

Results of wire beam electrode measurement
According to the anodic and cathodic reactions, the corrosion process is shown in Fig.5. For the N 2 and CO 2 and moisture atmosphere continuous injection, it corresponds to a variety of atmospheric environments with N 2 , CO 2 and O 2 in the experimental environment.N 2 and O 2 dissolve intoH 2 O is process, while CO 2 dissolution is both physical and chemical process. So that, CO 2 reaching the solution interior is much more easily than O 2 . In the reaction process, the edge of droplet is the thinnest area, and forms a thin electrolyte film because of its shape. It is more easily that oxygen permeates into the droplet and reaches the surface of the substrate. So the edge of droplet is oxygen-rich and presents cathode characteristic [7] . The area beside the edge of droplet has low oxygen content and high CO 2 content, so it presents anode characteristic. At the same time, the center of electrode has a relatively thick electrolyte film, and solution is origin without CO 2 and its reaction. So that it also presents cathode characteristic, explaining why it can form the structure of "cathode-anode-cathode ring". CO 2 dissolved gradual increase as corrosion reaction going, and concentration diffusion dominates the corrosion reaction process. H 2 CO 3 diffuse into droplet interior, and H + will become the dominant anion in depolarization reaction. Corrosion scale of FeCO 3 blocks on the electrode surface of in large quantities as corrosion reaction going, and hinders the mass transfer process of O 2 further. Therefore, the corrosion on the surface of the electrode becomes independent reaction in the later stage, and the difference between the cathode and anode regions is not obvious.

Conclusions
The dew point corrosion reaction under a droplet in acid atmosphere can form the "cathode-anodecathode ring" structure from the outer edge to the center under droplets, and the anode region corrosive more serious. Cathode current at the edge of WBE disappear gradually as the corrosion time was going, but the time of reaching stationary current density on electrode surface in 50% CO 2 is more rapid than it in 5% CO 2 atmosphere.
The results point to that the reaction gradually transferred to the uniform corrosion on electrode surface when the dew point corrosion reaction reaching the late stage, and the characteristics of the galvanic reaction of the electrode decreased gradually. It comes from the dissolution, diffusion and reaction of gaseous corrosion medium of CO 2 and O 2 .